Neurophysiology

Cortical activity
There are regional differences in cortical activity in REM sleep. The dorsolateral prefrontal cortex is particularly inactive. Unlike NREM sleep in which the thalamic 'gate' partially disconnects the cortex from the brainstem, the cortex still reacts to sensory information but in a different way than during wakefulness. The exception is that there is little activity in the spinoreticular tract which mediates pain, and painful sensations are very unusual in dreams.

There is evidence for interhemispheric disconnection compared to wakefulness, but similar to that which is seen after sectioning of the corpus callosum.

Transcranial magnetic stimulation has revealed this disconnection particularly in the frontal and central areas of the cortex compared with the parietal and occipital regions [8]. It may be responsible for some of the lack of insight during dreams, time distortion and failure of some subjects to report dreams.

Muscle atonia
The pontine reticular formation ventral to the locus coeruleus (peri-locus coeruleus), including the LDT/PPT nuclei, projects to the nucleus magno-cellularis and nucleus paramedianus in the medial medullary reticular formation via the lateral tegmento-reticular tract (Fig. 2.2). Glutamate, which acts on receptors of the NMDA type, and acetylcholine are released and excite neurones in these nuclei. They project via the ventrolateral reticulospinal tract to the alpha motor neurones. The medial medullary centres are also activated by the locus coeruleus and the red nucleus which thereby reduce muscle tone. The motor neurones in the cranial nerve nuclei and spinal cord are inhibited by release of glycine, and this leads to the intense muscle atonia which is characteristic of REM sleep.

The continuous activity of the cerebral cortex is transmitted to the basal ganglia which remain active in REM sleep, but the complex behaviour patterns that they formulate are not enacted because of the intense inhibition of skeletal muscle tone.

Phasic activities
Many of the manifestations of REM sleep occur irregularly and apparently spontaneously due to intrinsic activity of the brainstem mechanisms responsible for REM sleep. These phasic activities include:

Muscle atonia
The pontine reticular formation ventral to the locus coeruleus (peri-locus coeruleus), including the LDT/PPT nuclei, projects to the nucleus magno-cellularis and nucleus paramedianus in the medial medullary reticular formation via the lateral tegmento-reticular tract (Fig. 2.2). Glutamate, which acts on receptors of the NMDA type, and acetylcholine are released and excite neurones in these nuclei. They project via the ventrolateral reticulospinal tract to the alpha motor neurones. The medial medullary centres are also activated by the locus coeruleus and the red nucleus which thereby reduce muscle tone. The motor neurones in the cranial nerve nuclei and spinal cord are inhibited by release of glycine, and this leads to the intense muscle atonia which is characteristic of REM sleep.

The continuous activity of the cerebral cortex is transmitted to the basal ganglia which remain active in REM sleep, but the complex behaviour patterns that they formulate are not enacted because of the intense inhibition of skeletal muscle tone.

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Phasic activities
Many of the manifestations of REM sleep occur irregularly and apparently spontaneously due to intrinsic activity of the brainstem mechanisms responsible for REM sleep. These phasic activities include:

Functional neuro-imaging
In REM sleep the pons and areas corresponding to the ascending reticular activating system are active. The dorsolateral prefrontal cortex is completely inactive as in stages 3 and 4 NREM sleep, but in contrast several areas of the limbic system are highly active.

These include the anterior cingulate gyrus amygdala, lateral hypothalamus, the orbitomedial prefrontal cortex and the parahippocampal gyrus. Inactivity in the dorsolateral prefrontal cortex, which is involved in executive and planning functions, probably underlies the illogicality, but retention of social awareness, which is characteristic of dream mentation.

The overall metabolic rate of the brain in REM sleep is the same as in wakefulness, but greater than in NREM sleep.

Electroencephalogram (EEG) activity
The EEG reflects the intense cerebral cortical activity that distinguishes REM from NREM sleep, and its similarity to the EEG of wakefulness led to the term 'paradoxical' sleep for REM sleep. The depth of REM sleep is hard to quantify except perhaps through the REM density and the duration of REM sleep. The characteristic electrophysiological features of REM sleep are a combination of a wide range of 'desynchronized' EEG frequencies, loss of EMG activity and the presence of rapid eye movements.

Mental activity
REM sleep is characterized by loose mental associations, and the cortical processes responsible for these and for dreams are activated primarily by intrinsic activity within the pontine centres rather than by external or internal stimuli as in wakefulness. Mental activity in REM sleep is reflected in dreams which are characteristically full of activity, narrative and incidents, especially those occurring later in the night.

They differ from the more thought-like content of NREM sleep, and often, but not invariably, coincide with phases when rapid eye movements are present.

Motor activity
The intense inhibition of alpha motor neurones in REM sleep prevents dreams and other cortical processes from being enacted. Tone is only retained in certain essential muscles, such as those of the middle ear, diaphragm and the posterior crico-arytenoids and to a lesser extent the parasternal intercostal muscles.

Occasional jerking of the limbs during REM sleep represents a brief failure of inhibition of muscle tone and the rapid eye movements characteristic of REM sleep are a similar phasic phenomenon. These occur particularly during dreams, but also at other times during REM sleep, and may be accompanied by bursts of small movements of the facial, limb and trunk muscles.

Autonomic function
Autonomic activity is extremely variable in REM sleep. In general, sympathetic activity is reduced and parasympathetic activity increased. The rapid fluctuations in their balance may coincide with changes in activity in the pontine reticular formation or possibly in the cerebral cortex. The limbic cortex, in particular, is able to influence the hypothalamus, which translates cortical activity into autonomic function.

Metabolic rate
The metabolic rate during REM sleep is similar to that during wakefulness while resting.

Temperature control
Homeostatic temperature control is grossly impaired in REM sleep.

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